1. Field of the Invention
The present invention relates to devices for measuring electrical current by converting current into a voltage. More particularly, the invention relates to high-bandwidth devices for measuring both direct and alternating currents that are isolated from the circuit being measured.
2. Description of Related and Prior Art
Several techniques for measuring current are known. The simplest one comprises a shunt resistor inserted into the current path. While this method is inexpensive, it provides no isolation, and it consumes excessive power, especially when a large signal amplitude is required.
Another technique employs a Hall-effect current sensor. This kind of sensor provides isolation, good resolution and low losses. Unfortunately, the bandwidth of such a device is limited to a few hundred kilohertz, and the cost is high compared with that of a shunt resistor. Moreover, this sensor exhibits a sizeable temperature drift, even when additional compensating circuitry is employed. Furthermore, to achieve an accuracy that is satisfactory for most purposes, the sensor must be implemented using a relatively large magnetic core with a gap in order to adequately concentrate the magnetic field around the current-carrying conductor. This adds to the bulk, weight, and cost of this type of current sensor.
Recently, current sensors based on the Anisotropic Magneto Resistance (AMR) effect have been introduced. The AMR effect results from the resistance change of ferromagnetic materials in the presence of an applied magnetic field. Using this effect, it is possible to build a sensor with a higher bandwidth than a Hall-effect sensor, but the output voltage of such a sensor is also subject to temperature variations. Furthermore, to generate a sufficient output voltage, an amplifier circuit is required that adds cost to the device. Overall, this sensor is more expensive than either a Hall-effect current sensor or a simple shunt resistor.
The Giant Magnetic Resistance (GMR) effect may also be used for current measurement. This effect results from the magnetic sensitivity of the resistance of a multi-layer structure. Although the GMR effect exhibits a higher sensitivity than the AMR effect, the GMR effect also exhibits a strong non-linear behavior. As of yet, no commercial current sensors based on this technique are available.
Fluxgate-based sensors have better accuracy and temperature behavior than sensors based on any of the above principles but achieve this at remarkably higher cost, and larger physical size. This makes such sensors undesirable for many applications.
Another class of sensors is based on the principle of routing a current to be measured through the primary side of a transformer and using it to hold the transformer core in saturation. A voltage is then applied at the secondary side to force the core into the linear range, creating a current through the secondary side that is proportional to the primary current. This secondary current is then measured using a sense resistor. However, such sensors are unable to measure small currents that are insufficient to saturate the magnetic core.
Currents near zero can be measured with the addition of a bias winding carrying a steady current sized to keep the core in saturation. However, the bias current produces a steady power drain, and furthermore produces a voltage offset at the sense resistor. Therefore, to achieve high accuracy, the current through the bias winding must be controlled to high precision. Furthermore, the bandwidth of this type of current sensor is necessarily quite small because it employs a magnetic core of a ferrite or permalloy material with a large cross-sectional area. In addition, because ferrite and permalloy cores are highly susceptible to temperature changes, this type of sensor exhibits a significant temperature dependence.
Accordingly, there is a need for an improved current sensor that measures alternating and direct currents over a large measurement range while exhibiting high-bandwidth and high-accuracy and achieving low power consumption, good isolation from the current being measured, low temperature sensitivity, and low cost.